Trichohyalin is a major differentiation product of the inner root sheath cells of the hair follicle, where it is initially deposited in large dense granules, termed trichohyalin granules. Subsequently, during terminal differentiation, it is dispersed and becomes associated with the keratin intermediate filaments of the inner root-sheath cells. Thus it seems to serve largely as an intermediate filaments associated protein in this tissue. It is also expressed in the medulla of coarser hairs where it does not interact with filaments, but rather changes into a loose amorphous product. Trichohyalin is of interest because it is a major substrate for transglutaminases, and it also undergoes postsynthetic modifications of certain of its arginine residues to citrullines. Of especial interest is the recent observation that trichohyalin is also expressed in the epidermis, but its role in epidermal differentiation and role in pathology remain to be elucidated. We have obtained a CDNA clone by PCR analysis of genomic DNA and have used this as a probe to isolate the human trichohyalin gene. The human trichohalin protein consists largely of a series of quasi-repeating peptides, and interestingly like profilaggrin, its amino terminus contains two functional calcium-binding domains of the EF-hand type. Our interest in this system is to explore its role as a transglutaminase substrate, as a major calcium-binding protein, as an important new intermediate filament associated protein, and its role, if any, pathology.